Latitude and longitude form a geographical coordinate system used for locating places on the surface of the earth. They are angular measurements, expressed as degrees of a circle measured from the center of the earth. The earth spins on its axis, which intersects the surface at the north and south poles. The poles are the natural starting place for the graticule, a spherical grid of latitude and longitude lines.


Halfway between the poles lies the equator. Latitude is the angular measurement of a place expressed in degrees north or south of the equator. Latitude runs from 0° at the equator to 90°N or 90°S at the poles. Lines of latitude run in an east-west direction. They are called parallels because they are equally distant from,

Lines of longitude, called meridians, run in a north-south direction from pole to pole. Longitude is the angular measurement of a place east or west of the prime meridian. This meridian is also known as the Greenwich Meridian, because it runs through the original site of the Royal Observatory, which was located at Greenwich, just outside London, England. Longitude runs from 0° at the prime meridian to 180° east or west, halfway around the globe. The International Date Line follows the 180° meridian, making a few jogs to avoid cutting through land areas.


A degree (°) of latitude or longitude can be subdivided into 60 parts called minutes ('). Each minute can be further subdivided into 60 seconds ("). One degree of latitude equals approximately 69 miles (111 km). One minute is just over a mile, and one second is around 100 feet (a pretty precise location on a globe with a circumference of 25,000 miles). Because meridians converge at the poles, the length of a degree of longitude varies, from 69 miles at the equator to 0 at the poles (longitude becomes a point at the poles). The diagram at right is an example of a place located to the nearest second. It is written as: 42°21'30"N 71°03'37"W. This place is city center, Boston, Massachusetts.


Which way up . . .
The north and south poles are the earth's geographic poles, located at each end of its axis of rotation. All meridians meet at these poles. The compass needle points to either of the earth's two magnetic poles. The north magnetic pole is located in the Queen Elizabeth Islands group, in the Canadian Northwest Territories. The south magnetic pole lies near the edge of the continent of Antarctica, off the Adélie Coast.
The magnetic poles are constantly moving.